In order to comply with the increasingly stringent emissions legislation, it is necessary to carry out an exhaust gas aftertreatment for reducing pollutants such as nitrogen oxides (NOx).
Such nitrogen oxides may be reduced, for example, with the aid of a urea-water solution, which is also referred to hereinafter as UWS, and a catalytic converter situated in the exhaust gas system of a vehicle operated using a diesel engine. In this method, the urea is converted into ammonia via thermolysis and hydrolysis. A so-called SCR (selective catalytic reduction) catalytic converter subsequently reduces the nitrogen oxides into water and nitrogen and thus reduces the pollutant component. This method is referred to as the SCR method.
For this purpose, it is necessary to equip these vehicles with an additional tank in which the urea-water solution may be stored. From this tank, the required quantity of the urea-water solution is conveyed to a device including a metering valve with the aid of a pump and corresponding supply lines and fed into the exhaust gas system, for example, by spraying. Such a metering device for reducing pollutants in exhaust gases is believed to be understood, for example, from DE 10 2008 008 564 A1.
Here, it is necessary to inform the driver about an instantaneous filling level in the tank of the urea-water solution via an indication device such as a display. Furthermore, it is necessary to prompt the driver to replenish the urea-water solution in a timely manner through the use of a suitable arrangement.
In addition, for legislative reasons, a warning scenario is initiated for the driver if the driver does not fulfill this replenishment request. In the extreme case, even restarting the engine is prevented. Therefore, there is the need to ascertain the quantity of the urea-water solution in the tank accurately and also to correctly detect a replenishment operation.
To ascertain a filling level in a tank of the urea-water solution, filling level sensors are used which are situated in the tank. There are variants in which the filling level sensor is situated on the upper side or on the lower side of the tank. In addition, there are continuously and discretely measuring sensors. The ascertainment of the fluid level in the tank is common to all variants.
In order to ascertain the actual quantity or the volume of the urea-water solution in the tank, there are essentially two presently used methods according to the related art.
According to the first method, a calculation of the volume of the fluid in the tank of the vehicle while standing is carried out with the aid of a filling level sensor, whereby a correction of the measurement may take place, for example, including position information generated by an ESP (electronic stability program) sensor system. In the case that the vehicle is inclined at the moment of the measurement of the filling level via the filling level sensor, this may be detected with the aid of the data provided by the ESP sensor system. In this case, the incorrectly ascertained filling level value is corrected with the aid of a correction-calculation algorithm, and the error during the filling level measurement is thus minimized.
Since, in many applications, the ESP sensor system either does not have sufficient accuracy, or it is not possible to perform a plausibility check in a manner sufficient to satisfy the exhaust gas legislation, the second method for ascertaining volume is usually the practice which is presently used, which is also referred to as the multisensor principle.
In this case, the sloshing behavior and the momentary inclination are modeled via the sensor signal with the aid of a filter, in order to prevent erroneous indications in these situations. Inclinations of the vehicle lasting a longer time constitute a particular difficulty for this method, as occur, for example, during ascents. For example, during a long ascent, a significantly higher or lower tank filling level may be modeled than which is actually present. The reason for this is the surface of the urea-water solution in the tank, which is always level, regardless of the position of the vehicle. In addition, during the ascent in the same example, a replenishment may thus be detected erroneously.
In this description, an ascent may be understood to mean a motion of the motor vehicle on an uphill grade for a longer period of time. The period of time is selected to be long enough that driving over smaller rises or hills which takes only a few seconds does not result in the detection of an ascent. Equivalently to the ascent, a descent may be understood to mean a motion of a motor vehicle on a downhill grade over a longer period of time.
To prevent such errors during the volume ascertainment, a piece of information about a longer ascent may, for example, may be used via a determination of the instantaneous air pressure. However, this model has great tolerances. During such an ascent, a correction may be made in the volume calculation with the aid of this air pressure information.
In the second method from the related art, to improve the accuracy of the calculation of the tank content of the vehicle, the tank content during travel is calculated via a variable, the instantaneous consumption, which is available in the vehicle. This piece of information is usually available in a motor vehicle both via the fuel used and via an agent for exhaust gas aftertreatment fed into the exhaust gas system.
The disadvantage of this calculation method is that the consumption calculation over a longer period of time has an error which is too large. To reduce this error, it is furthermore known to combine this calculation method according to the instantaneous consumption with a method for ascertaining the tank content with the aid of a filling level sensor.
In this case, the signal of the filling level sensor, which may be falsified by tilting and inclination influences, is corrected with the aid of a suitable correction algorithm with the aid of vehicle position information. An electronic stabilization system (ESP) may, for example, provide such position information.
Since, in the method of the tank volume calculation using instantaneous consumption information, the error in the consumption ascertainment increases with time due to the summation of small variables, a range is established around the result of this summation, which is referred to as a window below, within which the value of the actual tank volume is assumed.
If the signal of the filling level sensor is within this established range during a comparison, i.e., if the results of the calculations match according to the first method and according to the second method, this value is assumed to be true and the error is set to zero. Subsequently, the calculation of the instantaneous consumption is continued as of this value.
The resulting error growing with time is kept as low as possible during the instantaneous consumption calculation in combination with the corrected filling level sensor signal. In addition to an improvement in the accuracy of the tank gauge, which is in the range of plus or minus one liter, the influence of the dynamics of the tank content on the tank level sensor and thus on the tank gauge is eliminated via a calculation of the tank content with the aid of the instantaneous consumption values, but the plausibility is not essentially improved.